This invention generally relates to a method for microdeburring small bores, and is particularly concerned with the removal of microburrs from the timing and metering spill ports in the fuel metering barrel of a diesel engine fuel injector.
Methods for deburring the microbores that form the timing and metering spill ports in metering barrels are known in the prior art. However, before the shortcomings associated with such prior art deburring methods can be appreciated, some background with respect to the manufacturing and operation of such metering barrels is necessary.
Metering barrels form part of the fuel injectors of diesel internal combustion engines. The function of such barrels is to precisely meter a timed flow of diesel fuel through the fuel injectors, which in turn inject the fuel into the individual cylinders of the engine. To this end, such metering barrels include an elongated cylindrical body having an axially oriented bore (known as the plunger bore) through which a metering plunger reciprocates. The cylindrical wall of the metering barrel is radially traversed by both a timing spill port and a metering spill port which are spaced apart along the longitudinal axis of the barrel. Both the timing and metering spill ports are formed from small, laser-drilled holes only about 24/1000 of an inch in diameter that intersect with the plunger bore that forms the inner diameter of the barrel. In operation, the metering barrel precisely meters and regulates a timed flow of diesel fuel to the cylinders of the diesel engine by means of a spool-valve type interaction between the head of the metering plunger that reciprocates through the barrel, and the small-diametered timing and metering spill ports that interface with the plunger bore.
In order for the metering barrel to effectively perform its function within the fuel injector, the diameter of the metering and timing spill ports at the point of interface with the plunger bore must not deviate significantly from the port diameter 24/1000. Unfortunately, during the manufacturing of the metering barrel, microburrs are formed around the inner diameter of the metering and timing spill ports at the interface of the plunger bore both as the result of the laser boring operations and the honing operations which are necessary to create a micro-finished surface within the barrel through which the plunger can reciprocate in a fluid-tight relationship. Because these microburrs have the effect of reducing the diameter of the metering and timing spill ports at their critical interface with the plunger bore, they must be removed.
In the past, these microburrs were removed by conducting a pressurized flow of abrasive liquid through the timing and metering ports. The liquid used might be formed, for example, from glass microbeads entrained in a silicone polymer-based liquid that is pressurized to 5000 psi and expelled from a nozzle. While such a pressurized, abrasive liquid has proven to be effective in removing the microburrs from such ports, the applicants have observed a number of shortcomings associated with this technique. Specifically, the applicants have observed that the use of such a pressurized abrasive liquid creates an unwanted widening or radiusing of the timing and metering ports in the area where they intersect the plunger bore, thus effectively increasing their diameter to a size substantially larger than the 24/1000 diameter necessary for them to accurately perform their task. Such widened or "radiused" bores causes the metering spill port to start conducting a flow of fuel earlier and to end this flow later than normal, thereby causing the fuel injector to inject excess fuel in its respective diesel cylinder. The excess fuel condition is not only wasteful of fuel, but increases the amount of diesel pollutants expelled out of the exhaust system of the engine. Additionally, the silicone polymer-based liquid carrier of the abrasive particles is difficult to completely remove from the metering barrel after the deburring operation has been completed. The unremoved particles of abrasive and removed metal can lodge between the inner surface of the barrel and the plunger, resulting in serious scarring of the barrel surface (which compromises the seal it makes with the reciprocating plunger), and sometimes even causing injector sticking. While a tubular shield may be interposed around the inner diameter of the injector barrel during the deburring process in order to prevent the abrasive liquid from scarring the polished inner surface of the barrel, the high abrasiveness of the liquid abrasive mandates the use of heavy, thick walled shields that wear out quickly, and must be replaced frequently. The hydraulic equipment necessary to create and maintain the high pressure of the liquid abrasive is expensive, as is the silicone polymer-based carrier that forms most of the volume of the abrasive liquid. Finally, this equipment is difficult to start and stop within the small time period (i.e., a fraction of a second) that it takes for the resulting pressurized stream to microdeburr the bore, which is a factor which contributes to unwanted radiusing.
Clearly, what is needed is a method for microdeburring bores in a workpiece which is capable of completely and effectively removing such burrs without widening or radiusing the bores, particularly in the area where the bores interface a finished surface. Ideally, such a method should leave behind a minimum of abrasive residue, which in turn can be completely, easily and precisely removed from the workpiece. Finally, such a method should be easily and precisely controllable so that excess abrasive action can be avoided, and implementable by relatively inexpensive equipment.